A GSM network consists of multiple entities. FIG. 1 shows the general structure of a GSM network. A Base Station Subsystem (BSS) controls the radio link to mobile stations (MS) of subscribers including a Subscriber Identity Module (SIM). The Base Station Subsystem consists of Base Transceiver Stations (BTS) and Base Station Controller (BSC). They communicate via standard Abis-interface. A Network Subsystem, main part of which is a Mobile services Switching Center (MSC) performs connecting and routing of calls between mobile users or between mobile and fixed telephony users. Mobile station and BSS communicate over a Urn-interface (air interface, radio link) while BSS communicates with MSC via an A-interface.
The Base Station Transceiver defines the cell and controls the radio link to mobile stations. The BSC controls the resources of one or more base stations, call setup procedures and different GSM radio link features like frequency hopping, power control, handover algorithms etc. The BSC is the connection between mobile station and MSC. Um-, Abis- and A-interfaces are standard interfaces organized in three (3) layers. Different procedures, like call setup, call release, handovers, power control procedures, etc., are performed and messages on layers 1, 2 and 3 are exchanged between mobile station and base transceiver station (BTS), BTS and base station controller (BSC) and BSC and mobile switching center (MSC). The BSC controls radio link parameters and call procedures (power control, handovers, call reestablishment, etc.) mainly using messages, counters and timers to control the flow of different procedures. Mobile and base station measurements and messages on Um-, Abis- and A-interfaces can be recorded in form of counters on BSC. The Network Subsystems comprises several databases, i.e. Home Location Register (HLR), Equipment Identity Register (EIR), Authentication Center (AuC) which contain information about subscribers, equipment and subscriber authentication.
After a new GSM site is built and covers a certain area with a radio signal, the situation concerning the telecommunication traffic is static. Traffic is handled by this base stations whose downlink signal strength received by the mobile station is the greatest. Uplink is not considered. Nowhere in a GSM system exists information regarding a receiving antenna branch problem (inherent for GSM standard and regardless of vendor), e.g. in form of an alarm which is visible for a maintenance team. Uplink problems can be caused by damaged connectors, water in cable or connector, feeder too long, etc. The strength of the signal transmitted by a mobile station towards the receiving branch of a serving base station is not taken into consideration during call setup and handover (change of the serving radio cell). Note: we are not discussing the old serving cell (handover). There is no information about uplink strength and quality for a new target cell. There is no uplink information until one already made handover or call.
Further, there are always changes in customers' behavior in certain geographical area due to seasonal migrations, road opening, road closing, new buildings blocking GSM signal, new shopping mall, etc.
Base station capacity is dimensioned according to information in the past. A new situation can spoil the previous “harmony” making some base stations overloaded and others underutilized. In reality it is very hard to monitor all traffic changes. It takes a lot of manpower to monitor and when an overload is spotted it is difficult to handle.
Performance Measurement Overview
Measurement Categories and Types
As an overview, base station system measurements can be grouped into different categories.                Radio resource related measurements: traffic channel (TCH), signal channel (SDCCH), control channel (CCCH) related measurements.        Procedure related measurements: immediate assignment, assignment, handover related measurements, etc.        Quality measurements: idle channel measurements (interference measurements), busy channel measurements (signal level and quality measurements).        Equipment related measurements: processor load, availability measurements, Abis-interface, A-interface, SS7 related measurements.        GPRS related measurements.        
For example, before and during every connection (signal and traffic channels), both mobile and base station that is a serving cell for the call perform measurements of signal level (RXLEV) and bit error rate (BER) that is mapped to discrete RXQUAL levels according to table 1. RXLEV and RXQUAL measurements are averaged and send from mobile station to serving base station every SACCH multiframe (104 TDMA frames, approximately 480 ms). Those measurements are important for cell selection/reselection and handover procedures and are also an important indicator of speech quality and coverage issues in the cell area.
TABLE 1Bit Error Rate (BER) to RXQUAL relationRXQUALBER0<0.2%1<0.4%2<0.8%3<1.6%4<3.2%5<6.4%6<12.8%7>12.8%Overview of Performance Measurement Tasks
There are several main reasons why the performance measurement (PM) is necessary part of any GSM network surveillance system:
Error Detection and Correction
Error sources as for example interference are hardly foreseeable during the network planning phase. Conclusions and feedback about the actual situation in network can be done with performance measurement. Error detection can be done from the point of view of mobile subscriber and from technical point of view. The most interesting performance indicators are:                Immediate assignment success rate        Assignment success rate        Call setup success rate        Handover success rate        Call chip rateTraffic Load Measurement        
The measurements of cells' traffic load are the basic information required for improvements and expansions of the offered calls. The following measurements are of great interest:                TCH traffic load (traffic offered, traffic carried, traffic lost due to congestion)        SDCCH traffic load (same as TCH traffic, for signaling channels)        Paging channels congestion        Access grant channel congestion        CCS7 load        Number of call setups        Number of handovers        Number of location updatesResource Availability Measurements        
It is possible to find out if all of the originally installed resources are available or partly available. If, for example, some of traffic channels (TCH) are not available that could lead to congestion of other TCHs of the cell and unsuccessful setups of new connections. The following resources are most important:                Traffic channels (TCH)        Slow dedicated control channels (SDCCH)        BSC, BTS, TRX availability        SS7 links        PCM connectionsQuality of Service (Qos), Grade of Service        
QoS is extremely important for the network operators. It is required to provide the mobile subscribers good speech quality and good grade of mobile services. By analyzing the measurement data, performance measurement can provide data about average seizure time of signaling and traffic channels allowing the conclusions about the duration of different procedures (logging into a cell, setup of a connection, etc.). Also very important measurements are TCH loss, handover loss, drop call and measurements of the cell's interference intensity.
Statistical data: For the network marketing and financial departments the data about participants' behavior are of the utmost importance. Different approaches to traffic analysis, different regional approaches in network planning and utilization and need to evaluate network functionality and performance objectively require mid- and long-term statistical analysis of the PM data.
The goals of PM are dependant on the tasks of different groups of users:
The Network Planning
Network planning is done under aspects of costs, geography, traffic density, traffic model, subscriber behavior. PM is used to support and evaluate the following:                verification of network configurations for future planning        evaluation of traffic density for planning the network growth        resource access        resource availability        quality of service        
PM helps plan the size of location areas (determining the number of pagings and location updates), the number and configuration of cells, cell parameters influencing the QoS, hardware equipment used etc.
Network Operation
Network operation performs network online supervision, fault detection, fault clearance and troubleshooting, resource availability etc. Number of dropped TCH/SDCCH connections, TCH/SDCCH congestion, handover drops, call success rate, processor loads and other indicators are monitored.
Network Optimization
Network optimization is responsible for improvement of network performance, cost efficiency and network quality from technical and customers' points of view. The main goals of PM are the improvement of:                Grade of service        Quality of service        Performance and utilization        Availability        Survivability        
PM is used to detect and resolve problems with network congestion, network errors, load problems, interference problems, capacity problems, network quality problems.
Marketing and Controlling
Marketing and controlling are planning the new services and evaluate the usage of existing. The main goals of PM are to provide data about:                usage of services and features (utilization)        customer behavior (regional, seasonal and daily differences in network usage)        
Different GSM network subsystems provide measurements regarding performance of the subsystem. The most important for the listed tasks are BSS performance measurements and MSC (switching subsystem) measurements. MSC performance measurement (MSC statistics) is not a part of the subject BSS performance measurement system.
Every hardware equipment vendor provides the system for collecting the counters and measurements and storing the “raw” data in some format. For example, equipment, measurement reports (or logs) are collected and stored on BSC disks. Operators decide which counters are of interest and collect such information by creating so called scanners. Measurement is defined with given granularity that is commonly 1 hour in GSM networks (all statistics are on hourly basis). These measurement reports are transferred to an Operation and Maintenance Center of the Base station system (OMC-B) via X25 interface (procedure of measurement upload) and are stored in a database that is part of OMC-B. The measurements can be converted and exported in ASCII file that can be processed using different tools for performance measurement processing that are either available on market or are in-house solutions developed by GSM operators. Since the whole GSM network is controlled by several OMC-B systems and the vendor specific software for measurement processing provides only very basic functionality with no statistical analysis, it is necessary to collect, store and process measurements in order to provide the statistics for the whole network.